The present invention relates, in general, to work holding devices, and more particularly to a device for holding the leads of a semiconductor device lead frame during wire bond operations wherein the leads of the lead frame are electrically connected to the bonding pads of a semiconductor device by bond wires.
One common method of packaging semiconductor devices is to attach the device to a metallic lead frame, electrically connect the device to the leads of the lead frame with bond wires, and then encapsulate the resulting assembly in a molded plastic body. The lead frame generally consists of a rectangular pad, or flag, to which the semiconductor device is attached, normally by either a form of solder or a form of epoxy. The flag is surrounded by a number of leads which are electrically isolated from the flag. The leads extend outward radially from the flag, and are joined together in a supporting structure that forms the lead frame. The semiconductor device has small electrical contacts about the periphery of the device which are bonded via filament wires to the leads of the lead frame, providing for electrical access to the device. The operation wherein the bonding pads are connected to the lead frame leads, called wire bond, normally consists of ultrasonically scrubbing a wire, with a diameter in the range of 25 to 125 microns, against the point to which the wire is to be bonded. The metal of the wire alloys with the metal of the bond point, forming a good electrical and mechanical connection. Since the leads are not mechanically supported on the ends to which the bond wire is attached, it is necessary during this wire bond operation to clamp the leads so that good electro-mechanical contact is achieved.
In the traditional lead frame, the flag was integrally connected to the lead frame. As circuit complexity increased, so did power dissipation of the device. It became necessary to increase the size of the flag so as to act as a more efficient heat sink for the device. As the flag was made larger, the length of the bond wires increased, becoming a limiting factor as to how large the flag could be made. It became necessary, for certain applications, to make the flag larger than was possible in the traditional configuration. Accordingly, an approach was developed wherein the lead frame was made without an integral flag. A large flag was then attached to the leads of the lead frame by means of non-conductive tape. The die was then attached to the flag, and the entire assembly processed through wire bond.
To form a good alloy during wire bond, it is necessary to perform the ultrasonic bonding at an elevated temperature. Therefore, the lead frame assembly with attached flag and semiconductor device was placed on a heater block and heated to the appropriate temperature. The leads were then clamped in the traditional manner. The problem was that the leads were not held between the clamp and the heater block as with the traditional lead frame configuration. Rather, the leads were held between the clamp and the attached flag, which rested upon the heater block. The tape cementing the flag to the leads became soft at the elevated temperature. The leads could then move in response to the ultrasonic energy, thus absorbing the ultrasonic energy. As a result, the mechanical integrity of the bond was reduced. Since the various leads could be of somewhat different thicknesses, the force of the clamp was different from one lead to the next, further allowing some leads to move while others were clamped effectively. If more pressure were used in order to clamp the leads more tightly, certain leads would be clamped too tightly. This would cause them to deform, and delaminate from the flag to which they were taped.
Another characteristic of the traditional wire bond lead clamps was that for different sized lead frames, different sized clamps were necessary. Thus, if the device that the wire bond machine was set up to work on was to be changed, it was necessary to remove the clamp, replace it with the appropriate new clamp, and realign the system. This could be a time consuming project, interrupting the efficient flow of work through the wire bond station.